Such gas measuring devices are known in various embodiments such as in the form of a detecting tube in which a forward layer splits up the gas to be detected into such components which lead to a change in color in the indicating rearward layer. U.S. Pat. No. 4,259,287 discloses a detecting tube wherein such a forward layer is made of a carrier material impregnated with ammonium chloride and a rearward indicating layer is made of a carrier material impregnated with an acid and bromphenol blue.
Other gas measuring devices such as disclosed in U.S. Pat. No. 4,052,162 have a colorimetric indicator tape which is sensitive for a specific type of gas. In order to be able to measure also other gaseous harmful substances with the same indicator tape, the tape is exposed to gas by means of a pretreatment tube wherein the gas component to be detected is converted into indicatable substances. Accordingly, to detect vinyl chloride, the known gas measuring device includes a pretreatment tube wherein the vinyl chloride is oxidized and chlorine is released which can be indicated by the chlorine-sensitive indicator tape.
In addition, detector tubes are known which can be utilized to specifically detect chlorine or bromine and whose reagent comprises ortho-tolidine. In this connection, reference may be had to the German publication "Prufrohrchen-Taschenbuch", Dragerwerk AG, Lubeck, May, 1985, page 49. A further known testing tube for liquid hydrofluoric acid or volatile fluorides contains as a reagent iron rhodanide with an additive of hydrochloric acid as disclosed in German Patent No. 1,000,170.
None of the known gas detecting devices can detect free fluorine although the limit values of the workplace concentration are set.